Sinclair’s aim with the ZX80, was to produce the lowest cost, but fully functional home computer that was possible. Hence every single part of the design and manufacturing was geared towards this.

Apart from the Z80, the ROM chip and the RAM chips, all the remaining chips were inexpensive standard 74LSxxx series parts.

The ZX81 was designed to lower the cost further. Hence the ULA replaced ALL the standard 74LSxxx series parts. A keyboard made of a plastic membrane also lowered the cost.

This left the ZX81 with a smaller (and cheaper) PCB, with only four chips or only five chips. The chips are:

Z80A or more likely a NEC unlicensed clone (NEC D780C-1) CPU

ULA

Mask ROM

1K byte SRAM chip

or

Two 1K by 4 bit SRAM chips (combined giving 1K byte)

So why 1K? Because this was the most cost effective SRAM chip available at the time that provided the minimum functionality that suited Sinclair’s requirements.

At that time, all RAM was very expensive. SRAM was very simple to use, and did not need any support chips. Whereas DRAM chips for larger amounts of RAM were cheaper per byte, but needed support chips. Complexity that Sinclair did not want.

To see an indication of how much DRAM cost, look through adverts in the magazines of the time and compare the cost of a ZX81 and that of a 16K RAM pack for it.